Add a pretty horrible hack to prevent clang from crashing with inconsistent PCH

Add a pretty horrible hack to prevent clang from crashing with inconsistent PCH
files.
 - The issue is that PCH uses a stat cache, which may reference files which have
   been deleted or moved. In such cases ContentCache::getBuffer was returning 0
   but most clients are incapable of dealing with this (i.e., they don't).

   For the time being, resolve this issue by just making up some invalid file
   contents and. Eventually we should detect that we are in an inconsistent
   situation and error out with a nice message that the PCH is out of date.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90699 91177308-0d34-0410-b5e6-96231b3b80d8